1. Technical Field
This invention relates to the technology of computer-generated numerical control (NC) of cutter tool paths for complex surfaces, and particularly to gouge avoidance systems that attempt to eliminate intersurface and intrasurface interference.
2. Discussion of the Prior Art
NC tool programs produced by CAD/CAM systems almost never cut parts right the first time. The NC tool program must usually run several test parts for debugging and modification before letting the program cut metal. This debugging is acceptable for industries which use NC programs to make production parts. It is uneconomic for companies trying to use NC to make molds or dies which are usually produced in lots of one, or at most two or three.
NC tool path programs using CAD/CAM systems do not produce correct tool paths because the print of the revolving cutter head is never the same shape as the surface to be duplicated or created in complex molds or dies. The print of the revolving cutter head must be moved along to remove metal leaving only the surface to be duplicated or created as the ideal result. Such result is never physically attainable, but can be approached with a higher degree of success than that now possible by prior art techniques. Deficiencies of the prior art can be attributed to either the NC tool program being too elementary and thus inaccurately rough in the guidance of the cutter head, or the program is extremely complex based upon sophisticated mathematical algorithms which estimate numbers with floating point coefficients and thus eventually produce errors.
The tool path generation program behaves, in the case of the elementary method, as if it were a blind caterpillar inching its way over each segmental surface of the part. The surface is represented usually as coordiantes of a mesh of a limited number of points in space. But consider that die cavities for automotive panels consist of several, many times more than 200, segmental surfaces. Like the blind caterpillar, the CAD/CAM system generating a tool path on one surface is unaware that the other segmental surfaces exist. When the tool reaches the edge of one surface, the path generation program is not smart enough to stop the tool's motion before it gouges the adjacent surface. Simplistic tool path generators do not have the ability to "look ahead" to see adjacent surfaces approaching. They know the diameter of the revolving cutter head but do not stop the tool before it collides with the adjacent surface.
The more complex program may consist of mathematical equations of a surface in space by analytical geometry. This method requires a minimum of input data, but the entire surface is dependent upon the accuracy of the analytical mathematical equations. These equations should simulate the real surfaces of interest and at the same time be manageable. This demands compromises between versatility and complexity. Estimations through floating point coefficients are involved in such techniques and are quite evident when reading examples of this state of the art (see U.S. Pat. No. 4,558,977; "Geometric Simulation of Numerical Control Machining", R. B. Jerard et al, ASME International Conference on Computers In Engineering, Aug. 1-3, 1988; "Simulation of Numerical Control Machining of Sculptured Surfaces", R. B. Jerard et al, International Symposium on Automotive Technology and Automation, Flims, Switzerland, October 1986). It is the floating point estimation and complex equations that prevents the equations from attaining absolute objective accuracy.
The prior art has attempted to solve gouge avoidance by essentially two techniques: (a) manual interference checking which is tedious, time-comsuming, and prone to costly errors; or (b) use of additional algorithms which attempt to compute intersections using floating point computations between the cutter and the surface (see U.S. Pat. No. 4,789,931; and "Development of an NC Machining System for Stamping Dies by Offset Surface Method", Sakuta et al, Conference Proceedings, November 1987"; and "Development of Complex Free Surface NC Milling System for Stamping Dies", Ikeda et al, Toyota Engineering, Vol. 38, No. 1, 1988, pp. 64-75). In these latter references, the revolving cutter has one mathematical model and the surface to be emulated another. The added intersection algorithms are additionally complex, time-consuming, and, most importantly, lead to roundoff and tolerance errors which cannot be avoided. None of these techniques provide for absolute assurance for avoiding gouge interference.
It is therefore an object of this invention to provide a method of generating NC cutter paths which provide for absolute intersurface and intrasurface interference protection for an unlimited number of segmental surfaces with any shape of cutter. With such absolute assurance, a physical or test piece no longer has to be made; the cost and time required for generating NC surfaces is reduced, and the accuracy, quality, and repeatability of cuts is improved. This system will not require a user to identify certain surfaces as drive or check surfaces, a labor intensive and error prone process.